Pulmonary arterial hypertension (PAH) is characterized by vascular obstruction and variable presence of vasoconstriction, leading to increased pulmonary vascular resistance and right ventricular failure. Despite its profound clinical consequences with high morbidity and mortality, therapeutic advances over the past 20 years have modestly improved survival of PAH patients. The combined use of available drugs to maximize the clinical benefit is an emerging strategy for the treatment of PAH. Pulmonary vascular endothelial nitric oxide (NO) synthase (eNOS)- derived NO is the major stimulant of cyclic guanosine 5'monophosphate (cGMP) production and NO/cGMP-dependent vasorelaxation in the pulmonary circulation. Alternatively, pulmonary vascular cGMP levels can also be elevated via inhibition of phosphodiesterase type 5 (PDE5) to achieve a similar response. We recently reported that an eleven amino acid (SSWRRKRKESS) synthetic peptide (P1) stimulates the catalytic activity of eNOS and causes NO/cGMP-dependent sustained vasorelaxation in isolated pulmonary artery (PA) segments and in lung perfusion models. More recent preliminary data suggest that P1 stimulation increases intracellular NO release and inhibits cGMP-specific PDE5 in pulmonary artery endothelial cells. Based on our earlier report and preliminary studies we hypothesize that P1 treatment with its unique dual action as a NO-releasing PDE5 inhibitor can be used as a novel therapeutic approach for the treatment of PAH. To test this hypothesis, the present study is designed to: 1) develop a physiologically relevant preclinical animal model of PAH with specific focus on assessment of bioavailability, dosing regimen, and efficacy of P1 therapy on the hemodynamic, histologic, and biochemical changes, and 2) identify the specific molecular events associated with P1-mediated caveolin-1/eNOS dissociation and activation of eNOS as well as the effects of P1 on PDE5 activity and cGMP hydrolysis. We anticipate that confirmation of mechanism-based dual action of this novel therapeutic peptide in a preclinical animal model will facilitate Phase I clinical trials for P1 treatment of PAH.